1. Field of the Invention
The present invention relates to an automatic fuel assembly-replacing apparatus which is installed in facilities, such as a nuclear reactor, a fuel storage pool or the like, so as to transfer nuclear fuel assemblies. More particularly, the invention relates to a nuclear fuel assembly-replacing apparatus suited for automatically positioning and transferring nuclear fuel assemblies in a facility in which the storage positions of the fuel assemblies are variable within a predetermined range.
2. Description of the Prior Art
A conventional fuel assembly-replacing apparatus is disclosed in, for example, Japanese Unexamined Patent Publication No. 52-135992. In the conventional fuel assembly-replacing apparatus for transferring fuel assemblies in a reactor or a fuel storage pool, the position of a fuel assembly is set at an X-Y coordinate on the X-Y axis plane. For positioning a fuel grapple to the set position, synchronous transmitters are rotated by intermesh between rack gears attached to the lateral surfaces of rails and pinions mounted on a bridge and a trolley. Based on output signals from the transmitters, the current positions (X and Y coordinates) of the fuel grapple are detected, and automatic control is performed on the bridge motor and the trolley motor so that the deviation between the current position of the fuel grapple and the targeted position of the fuel assembly becomes zero. This conventional technique is applicable only to the case where the targeted position of the fuel assembly is fixed and is unable to perform an automatic positioning in the case where the targeted position is variable within a specified range.
FIG. 6A is a cross sectional view illustrating the core of a boiling water reactor. Reference numeral 13 designates nuclear fuel assemblies; 29 denotes control rods; and 28 indicates a LPRM (Local Power Range Monitors). As shown in FIG. 6B, four fuel assemblies 13 surrounding one control rod 29 constitute a cell. Respective handles 13A of the four fuel assemblies 13 face the control rod 29.
While the reactor is operated, the reactivity of the core is gradually decreased. It is thus required that a used fuel assemblies be replaced by new ones at regular intervals. The operation period between fuel replacement times is referred to as a cycle. There are approximately 800 fuel assemblies in the core of a 110 million KW power reactor, and it is necessary that one-fourth to one-third of the 800 fuel assemblies be replaced at every periodical inspection.
FIG. 7 illustrates the loading pattern of the nuclear fuel assemblies, that is, the arrangement of the fuel assemblies, in the core of the boiling water reactor. Referring to FIG. 7, the figures (4-6) given to the respective fuel assemblies 13 denote the number of cycles while the assemblies remain in the reactor core. The assemblies with the same figure should be replaced by new ones at the same time. In FIG. 7, the fuel assemblies 13 with the cycle number of 4, 5 and 6 are designated by FIGS. 4, 5 and 6, respectively. The fuel assemblies 13 to be replaced at the same time, for example, the hatched fuel assemblies 13 with the cycle number of 6 are always obliquely aligned in a row and disposed adjacent to each other.
FIG. 8 is a perspective view illustrative of one example of the conventional nuclear fuel assembly-replacing apparatuses. Rails 1 are installed on the top floor of a nuclear reactor building. Mounted on the rails 1 are a bridge 2 on which a trolley 3 is mounted. A fuel grapple 4 for gripping fuel assemblies is mounted on the trolley 3. A bridge motor 8, a trolley motor 9 and a fuel hoist motor 10 are provided to move the fuel grapple 4 to a targeted position. These motors cause the fuel grapple 4 to move to the targeted position and also to be positioned thereto according to a signal output from an automatic motor controller 11 through a control cable 12. With the conventional technique performed by the above-described conventional apparatus, the fuel grapple 4 is positioned to a target position, that is, to stationary coordinates such as the reactor core or the fuel storage rack. Therefore, it is only essential that the nuclear fuel assembly-replacing apparatus detects the position of itself by the intermesh between the above-mentioned racks and pinions (not shown), thereby enabling the fuel grapple 4 to be positioned to the stationary target position.
However, the performing an automatic positioning of the fuel grapple 4 relative to carrying means which does not possess repeatability of its stop position, such as an underwater fuel assembly carriage, the above-described conventional technique is not operable to locate the fuel grapple 4 to a target position. This problem arises from the fact that, even though a reference stop position of the fuel assembly carriage is set to a target position, the carriage may actually be stopped at a position which deviates from the reference position, which makes it impossible to insert or remove a fuel assembly.